Field of the Invention
This invention relates to a digitally controlled, electronic tuner that can be tuned to any of several different channels at different frequencies and which includes an automatic fine tuning (AFT) circuit to adjust the tuning precisely when the frequency to which the tuner is tuned is within a predetermined frequency range close to the exact desired frequency. In particular, the invention relates to a television signal tuner for a receiver having an AFT circuit to maintain the tuning at a selected frequency according to one of the available channels and further including means to overcome the effect of the AFT circuit when it is desired to shift the tuner to a different channel.
U.S. Pat. No. 4,085,371 describes a television receiver tuner in which the basic tuning is accomplished by controlling the voltage on the one or more varactors in the circuits to be tuned. Varactors are capacitors, the capacitance of which is determined by the magnitude of the direct voltage applied across the capacitor terminals. The tuning circuit includes a memory section that retains information in digital form according to the frequencies to which the tuner may be set. Such information may be generated in the overall circuit and applied to the memory section by a user of the television receiver, normally at the time the set is first placed in operation. Television channels are commonly identified by number and there are usually not more than a dozen or so channels at any one location. Thus a receiving set incorporating the electronic tuner of the aforesaid Pat. No. 4,085,371 is likely to be adjusted to receive each of the available channels in turn when the set is first placed in use. Thereafter tuning is simply accomplished by actuating a switch according to the desired channel.
However, because of aging factors in the circuit or environmental factors that affect the receiver, the initial tuning information stored in digital form may become incorrect after the lapse of an indeterminate time. If the memory section always instructs the varactor control circuit to apply a certain voltage to the varactor or to the multiple varactors according to the initial setting memorized in the memory circuit, the receiver might eventually be tuned to a frequency different from the desired channel. The difference would become increasingly apparent as fuzziness of image or perhaps distortion of sound. Accordingly, the aforesaid U.S. Pat. No. 4,085,371 defines an AFT circuit that begins to operate when the frequency to which the tuner is set is within a certain range of the exact frequency for a channel. The range may be, for example .+-.1.2 MHz. This range is further divided into a sub-range just above the exact desired frequency and extending out to +1.2MHz above the desired frequency and another sub-range that extends from the desired frequency to a frequency lower by 1.2 MHz. If the tuner is operating so that the carrier of the received signal falls within the upper sub-range, the AFT circuit applies pulse signals to adjust the tuning downwardly to the desired frequency. Conversely, if the carrier of the received signal is in the lower sub-range, a different signal is applied from the AFT circuit to shift the tuning upwardly to the desired frequency.
While the tuner has been referred to as being "set" at a frequency, that is somewhat incorrect; it is to be understood that the setting may be changing virtually continuously as the tuner is adjusted from one desired frequency to another. For example, if the tuner is set to receive channel 2 and the user decides to switch to channel 5, the tuner will be required to leave the proper setting for channel 2, pass through the upper sub-range of that channel, and pass through the lower and upper sub-ranges associated with channels 3 and 4 in succession in order to reach the lower sub-range and then the desired precise frequency of channel 5. The effect of the AFT circuit must be overcome in order to keep the voltage produced by the AFT circuit from locking the tuner at the frequency of channel 2 or the frequencies of channels 3 and 4.